This invention pertains to a system for diagnosing the status of a nuclear reactor operating with breached fuel elements and more particularly to a system which diagnoses the status of a nuclear reactor operating with breached fuel elements which utilizes an "expert system".
The fuel in a fission type reactor is typically an isotope of uranium, such as uranium-235. The reactor fuel may take the form of a fluid, such as an aqueous solution of enriched uranium; but typically the fuel is solid, either metallic uranium or a ceramic such as uranium oxide or uranium-plutonium oxide. The solid fuel material is fabricated into various small plates, pellets, pins, etc.; which are usually clustered together and in an assemblage called a fuel element. Almost all solid fuel elements are clad with a protective coating or sheath that prevents direct contact between the fuel material and the reactor coolant. The cladding also serves as part of the structure of the fuel elements.
The operation of fuel elements generates heat, which heat is typically dissipated by means of a coolant passed through the reactor. The coolant can be water, operating as either liquid or steam, or the coolant can be a liquid metal, such as sodium or a sodium-potassium mixture. The coolant passes in proximate contact over the cladded fuel elements; and sound cladding isolates or separates the coolant from the radioactive fuel material. However, in the event of a breach in the cladding, the coolant directly contacts the fuel. The radioactive discharge may then, in turn, be conveyed via the coolant throughout the entire coolant system, thereby contaminating the entire system.
Also given off, as part of the radioactive discharge are isotopes, that not only give off typical gamma rays of radioactivity, but also give off what are known as delayed neutrons. The delayed neutron emitters are soluble in liquid sodium (the coolant) so that they readily blend in with the coolant, should a fuel element cladding breach occur, and flow from the coolant throughout the system.
Therefore, it becomes readily apparent that the event of a fuel cladding breach must be taken into account when designing and operating a nuclear reactor. Not every occurance of a breached fuel element should trigger the automatic or manual shut-down of a nuclear reactor. In some instances, a reactor may be safely operated with breached fuel elements. Presently, liquid-metal cooled nuclear reactors (LMR's) exist which are licensed to operate with failed fuel. This mode of operation is typically referred to in the art as run-beyond-clad-breach (RBCB) operation. The current practice in most countries that have LMR programs is to set conservative shutdown limits on the magnitude of delayed-neutron (DN) signals coming from breached fuel.
It would be advantageous in RBCB operation to significantly relax the conservatism in DN shutdown limits without compromising plant-safety assurance. Significant advantage could be derived from a system which could discriminate between cladding breach events which lead to plant operational degradation which might challenge safety or radiological performance guidelines and those events which do not. Such a system would allow the continued operation of the reactor under a stable breached pin condition, which would significantly improve reactor availability.
A device called an equivalent recoil area (ERA) meter, which is a multiple detector DN monitoring station was previously developed for monitoring DN signals coming from breached fuel in LMR's. This device is disclosed in U.S. Pat. No. 4,415,524 entitled "Apparatus and Method of Monitoring for Breached Fuel Elements", issued to Kenny C. Gross et al., which patent is incorporated herein by reference.
During breached-fuel operation, the ERA meter makes available to the reactor operator quantitative diagnostic information relating to the condition and dynamic evolution of a fuel breach. The diagnostic parameters include a continuous reading of the ERA value for the breach (which is a measure of the relative size of the breach). The ERA meter also provides continuous readings of the sodium transit time, T.sub.tr, to the detector station and the isotopic hold-up time, T.sub.h, a measure of the effective aging of DN precursors between birth in the fuel and their release to the coolant.
Since the time that the ERA meter was originally conceived, it has been discovered that, contrary to earlier beliefs, the age of a DN signal is not constant with time. It has been learned from two recent experiments performed in the EBR-II reactor that the age of the signal can change spontaneously and frequently, even when all other reactor variables are at steady-state. The physical mechanism that initiates the changes in the isotopic hold-up time are still not fully understood. But the implications of a dynamically changing isotopic age are quite unsettling. It makes it virtually impossible for a human reactor operator to interpret and assess the safety significance of a changing DN signal.
The reason for this is that the magnitude of a DN signal is a sensitive function of the age of the signal. Although, the ERA meter will help mitigate confusion and ambiquity by providing the operator with a separate reading of the DN age, the age is only one of several system variables that can cause a DN signal to change. If a signal is increasing, for example, table 1 lists nine possible physical variations that could have caused the signal to increase. Of course, any two or more of these physical variations could be occuring simultaneously. A similar matrix of physical causes also exists which may explain a decreasing DN signal.
TABLE I ______________________________________ Possible Interpretations of an Increasing Delayed-Neutron Signal ______________________________________ 1. Increasing local fission rate 2. Increasing breach area 3. Increasing flow past source Depends on combination of age* and DN concen- 4. Decreasing flow past source tration in Na 5. Decreasing T.sub.h 6. Increasing flow rate in sample line to DND 7. New defect starting elsewhere (different pin or new location in same pin) 8. Change in dilation characteristics (e.g. at inlet scoop of bypass loop, or leakage component at assembly-facility interface) 9. Drifting DND characteristics (malfunction) ______________________________________ *Total age is a combination of holdup time, T.sub.h, and transit time, T.sub.tr
It would, therefore, not be possible for a human operator to combine readings from the ERA meter with readings from flow, power, temperature, and various electrical sensors and then mentally step through the complex conditional branching hierarchy that is needed to arrive at an unambiguous interpretation of a change in a DN reading.
Currently, full interpretation of these variables requires several days to weeks of detailed analysis by teams of specialists. The time needed for such interpretations is exemplified in well known cases that required several man-weeks of analysis for full interpretation, such as the TOPI-2 experiment, conducted by EBR-II, MST, and the PNC of Japan; breached assembly DE-9 that scrammed the FFTF in August 1984; the P4 experiment conducted by RAS; and the MOL-7C test by the French and Germans. Since a reactor operator must make an immediate decision whether to scram the reactor, continue reactor operation, or manually shutdown the reactor, the lengthy time periods required for the above referenced experiments are unacceptable.
Therefore, in view of the above, it is an object of the present invention to make available a system that provides the reactor operator with a very rapid identification of off-normal RBCB conditions.
It is another object of the present invention to provide an apparatus and method which will allow significant relaxation of the present conservatism in DN shut-down limits without compromising plant-safety assurance.
It is still a further object of the present invention to provide a system which makes available, to a reactor operator, on-line diagnosis and interpretation of a variety of interacting physical variables during exposed fuel operation.
It is yet another object of the present invention to provide a system which makes available, to a reactor operator, information needed to make proper decisions about technical-specification conformance of the reactor during RBCB operation.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations, particularly pointed out in the appended claims.